The subject matter disclosed herein relates to a pump gear arrangement and in particular to a pump gear arrangement having a bi-metal shaft.
Pump gears are used in a variety of applications such as in fuel pumps for turbine engines. The pump gear typically includes a shaft having a spur gear and a drive spline formed on one end. To reduce cost and weight, pump gears are typically formed from a single unitary material. The pump gear is typically formed for example from a single piece of tool steel alloy bar stock. In general, the drive spline is a relatively small feature that is formed using conventional machining practices allowing it to be integrally and cost effectively formed on the pump gear shaft.
The pump gear may also provide additional functions such as provisions for mounting and driving centrifugal pumps, electrical generators, and auxiliary fuel or hydraulic pumps. These additional functions required a longer pump gear which incorporates drive splines, keys, and threads to retain these auxiliary components. These features are difficult to machine in the wear resistant materials needed for the pump gear. In addition, the cost of the pump gear material is typically over ten times the cost of the tools steels used for shafts, splines, and threaded joints. Reduced cost for raw materials and machining of splines and threads can be achieved joining wear resistant pump gear materials to conventional tool steel.
Due to the desire to form the drive spline as an integral feature, the machinability of material often drives the material selection. However, the desire for good machinability conflicts with a desired wear resistance property for the spur gear. In applications such as aircraft engines, the pump gear is operating in low lubricity jet fuel at high temperatures. This environment has been found increase the wear on the spur gear. While a higher wear resistance material could be used, this would increase the size, weight and cost of the pump gear since the drive spline may not be easily machined using these materials due to its small size. As a result, the desire for a light and small pump gear impacts operational life and increases maintenance costs.
Accordingly, while existing pump gears are suitable for their intended purposes the need for improvement remains, particularly in providing a pump gear having improved gear wear resistance while maintaining machinability of the drive spline and threaded features to retain additional components integrated with the pump gear.